The present invention relates to apparatus for centrifugation and more particularly to improvements in centrifuge rotors.
Centrifuge rotors made in the form of a thin walled structure or hollow shell are well known in the centrifuge art. Many early centrifuge rotors were constructed of sheet metal and were designed to hold a plurality of sample containing test tubes. The development of modern high-speed centrifuges initiated a trend toward the use of high strength solid rotors machined from solid bars or forgings. There remains today, however, a class of moderate speed table top size analytical centrifuges which are suited to the use of shell-type rotors. In such applications, a shell-type rotor provides adequate structural strength and can be manufactured more economically than a solid machined rotor.
In its simplest form, a shell rotor may be constructed of only two pieces, a formed rotor body or "shell" and a hub which serves to couple the shell to a drive shaft. In such an arrangement the bottom end of the shell is open, thereby presenting at least two significant disadvantages.
A first disadvantage of a shell type rotor having an open bottom is that it has greater windage than a closed or solid rotor. Thus, for a given driving force, an open shell rotor cannot achieve as high a rotating speed. This is an important consideration in centrifuges having fixed or preset speed settings, especially where it is desired to employ various rotor types interchangeably.
The windage problem which has been described cannot be remedied satisfactorily by simply providing a closed bottom to transform an open shell rotor into a closed shell rotor. While it is true that a closed shell rotor would have windage characteristics generally similar to a solid rotor, it is not comparable in terms of mass and inertia. As a consequence, a closed shell rotor tends to accelerate more rapidly and reaches a higher speed than a solid rotor of similar size. Predictable centrifugation operations with mixed rotor types cannot be carried out, therefore, unless some type of electronic or mechanical speed sensing and governing means are employed. To control both the parameters of acceleration and steady state speed requires a fairly high level of complexity in the control apparatus which is undesirable from the standpoint of its cost and ultimate reliability.
A second disadvantage of an open shell rotor is that in order to use conventional centrifuge tubes, a tube holder must be employed to contain the tubes in the rotor. This is particularly true in the case where such tubes are made of glass and subject to breakage under the stress of centrifugal forces. In many small centrifuges, the drive system is not protected against the entrance of fluids and any spillage occurring in the rotor chamber may damage the drive system. In addition, it is not uncommon in the design of small centrifuges to rely upon the fan effect of the rotor to provide a cooling air stream to the motor. In such cases, openings may be provided in the rotor chamber to duct air to the motor, and thus the need for precautions against fluid spillage is obvious.
Accordingly, it will be seen that there is a need for improvement in centrifugation apparatus which is provided by the present invention as set forth hereinafter.